The cosmic microwave background (CMB) radiation provides the most precise probe of the largest structures of the universe. Now, however, a team from Case Western Reserve University in Cleveland, Ohio, and CERN has discovered surprising evidence that the largest-scale features of the microwave sky seem to be correlated with both the motion and the orientation of the solar system (D J Schwarz et al. 2004).

The tiny temperature variations of the CMB were discovered by the Cosmic Background Explorer (COBE) satellite more than a decade ago. Then, in February 2003, the Wilkinson Microwave Anisotropy Probe (WMAP) team published the analysis of their first year of high-resolution observations of the full sky. In a stunning manner, the results from WMAP confirmed the Standard Model of modern cosmology, with its key elements of a period of cosmological inflation and a composition of 5% baryons, 25% cold dark matter and 70% dark energy. One real surprise, however, was how WMAP showed that the optical depth for microwave photons is high, which implies an unexpected early onset for star formation.

A second look at the publicly available WMAP data reveals anomalies at the largest angular scales (> 60°). For example, the angular two-point correlation function vanishes at scales larger than 60° (as already seen by COBE, but largely forgotten). In Fourier space, the vanishing of the two-point correlation function at large scales is reflected by the smallness of the quadrupole and octopole moments. As we observe only one universe, it is possible to attribute these findings to bad luck (cosmic variance), although - taken at face value - the measurement does not agree with the expectation from inflation. In fact, the WMAP measurements contain more information. Angélica de Oliveira-Costa and colleagues studied the cosmic quadrupole and octopole and realized that both are very planar and aligned, i.e. all minima and maxima happen to fall on a great circle on the sky - another unexpected feature (de Oliveira-Costa et al. 2004).

Craig Copi, Dragan Huterer and Glenn Starkman of Case Western Reserve University then developed a method to assign 1 directions to the 1-th multipole (multipole vectors). While Starkman was on sabbatical at CERN, the team was joined by Dominik Schwarz, also at CERN at the time, to test the claims of de Oliveira-Costa et al. by means of multipole vectors.

To their surprise, the new method revealed at high statistical significance (99.9% CL) that the observed quadrupole and octopole are inconsistent with a Gaussian random, statistically isotropic sky (the generic prediction of inflation). They also looked for correlations with any known directions on the sky. No significant correlation with the Milky Way was found, but a strong correlation with the orientation of the solar system (ecliptic plane) and with its motion (measured as the CMB dipole) showed up.

A comparison with 100,000 skies generated by Monte Carlo shows that each of those correlations alone is unlikely at more than 99% CL. Therefore, there is strong evidence either of some systematic error in the WMAP pipeline (although in a preliminary analysis, the team is now discovering similar features in COBE maps), or that the largest scales of the microwave sky are dominated by a local foreground.

This finding has vast implications. It casts doubts on the cosmological interpretation of the lowest-1 multipoles from the temperature-temperature correlation and from the temperature-polarization correlation, and in turn on the claim that the first stars formed very early in the history of the universe.

Further reading

H K Eriksen et al. 2004 Astrophys. J. 605 14.
A de Oliveira-Costa et al. 2004 Phys. Rev. D 69 063516.
D J Schwarz et al. 2004 Phys. Rev. Lett. (in press), www.arxiv.org/abs/astro-ph/0403353.